At present, data transmission employs mainly the existing telephone network and is carried out by utilizing lines switched by conventional telephone exchanges as well as direct lines serving for point-to-point communication.
As telephone networks are not originally intended for data traffic, their hybrid utilization does not enable optimal data transmission from the viewpoint of either quantity or maintenance costs.
More particularly, the following disadvantages are encountered:
considerable limitation of the data-transmission speed;
long times in establishing the connection;
greater error ratios.
To overcome these disadvantages, a Public Data Network has been internationally recommended by CCITT (International Telegraph and Telephone Consultative Committee). The principal features of this network are as follows:
(1) High transmission speed.
(2) Brief times for establishing the connection.
(3) Assurance of very low error rate.
(4) Utilization of wholly digital networks employing for transmission, as well as for signaling, envelopes (formats) generally consisting of 8 bits, six of which relate to information and originate at the subscriber station while two bits are for service and are synchronously added to the data stream generated by the subscriber. The in-line transmission speed (gross bit rate) equals 4/3 times the standard subscriber's speed (net bit rate).
(5) Beaseband connection between subscriber-side DCE and the exchange.
The baseband connection is economically advantageous since it is known that a baseband modem is less complicated and expensive than a modem operating in the speech band, which is able to transmit only on a conventional voice channel (300-3,400 Hz).
A baseband connection, however, is not possible when the subscriber is separated from the nearest exchange by a considerable distance, e.g. more than 10 km, because line attenuation of data signals becomes excessive for such long distances. On the other hand, if the inconvenience of line attenuation is overcome by the insertion of intermediate regenerators, the system becomes very costly.
Moreover, a branched network designed for data transmission and operating in baseband, able to serve all subscribers, does not currently exist. This means that at present only subscribers very near the exchange could use the service suggested by CCITT for the Public Data Network.
If a remote subscriber connected to a conventional speech-band channel uses a net rate of, say, 2,400 bits per second, the expensive speech-band modems must operate in line, according to CCITT Recommendations, at a gross bit rate equal to 4/3 times the net rate, i.e. at 3,200 bit/sec; modems of this type designed to operate at that rate are not yet commercially available.